1. Field of the Invention
The present invention relates to extraction processes. Particularly, the present invention is concerned with minimization of undesirable entrainment of insoluble phases in solvents employed in extraction towers having sealed-sieve trays. Indeed, in its more specific aspects, the invention is directed toward minimization of entrainment of the asphalt phase in the deasphalted oil solution in a deasphalting tower having sealed-sieve trays.
2. Prior Art References
There are many extraction processes known in the art. Reference is made to the following U.S. Patents to mention just a few: Nos. 2,274,030; 2,123,642; 2,950,244; 2,669,539; 2,520,391 and 2,707,163. Of particular interest is U.S. Pat. No. 2,950,244 since it teaches a process for extracting a residuum petroleum fraction with a hydrocarbon solvent using a sealed-sieve tray extraction tower of the type with which the present invention is especially concerned.
Referring specifically to liquid-liquid extraction processes such as deasphalting processes commonly used for the separation of asphalt from residuum to produce a deasphalted oil, a light hydrocarbon solvent, such as propane or butane, is introduced into the bottom of an elongated vertical extraction tower having a plurality of perforated plates referred to as sealed-sieve trays. The light liquid phase flows upwardly through the extractor in countercurrent direction to the continuous heavy residuum or asphalt phase which flows generally downwardly through the extractor. The deasphalted oil-containing solvent solution is withdrawn from the top of the deasphalter and asphalt solution is withdrawn from the bottom.
To promote efficient liquid-liquid contact each sealed-sieve tray is provided with end weirs and a vertical downwardly extending plate. The end weirs permit the retention of a layer of the heavy phase liquid on the top surface of the tray prior to flowing to the next lower tray via a downcomer zone. A downwardly extending plate defines, with the side of the extractor vessel, a downcomer zone and together with the layer of liquid on the plate below seal the downcomer zone from the light liquid phase. Consequently the light liquid phase flows upwardly through the perforations in each of the trays and is thereby dispersed into droplets which then rise through the heavy continuous phase layer on top of the plate.
Trays are sometimes provided above the feed inlet in a refluxing zone in order to promote efficient contacting and separation between the upward-rising light liquid phase and either a downward-flowing wash oil or a reflux oil precipitated from the light phase by a temperature change at the top of the tower.
As is known, in the use of such towers small amounts of residuum and/or asphalt phase are entrained in the deasphalted oil solution leaving the top of the extractor. Inclusion of the residuum or the asphalt in the deasphalted oil, of course, greatly deteriorates the quality of the deasphalted oil. In the past, when the entrainment has become too severe, attempts have been made to reduce this entrainment, particularly at high feed and solvent rates, by feeding the residuum at a lower tray in the tower in order to provide more separation zones above the feed. Unfortunately this technique reduces the yield of the deasphalted oil since fewer trays are used for promoting extraction between the residuum and solvent phases. Thus there remains a need, particularly as flow rates are increased, for reducing undesirable entrainment of the insoluble phase in the solvent of solvent extraction processes, and particularly to avoid the entrainment of asphalt and residuum phases in the deasphalted oil-containing solvent in the extraction of residuum.